Probably Halo 3, I have no idea how many hours I would have spent on it, but it would have been a lot. Halo 3 also wins for the longest time spent playing a game without interruption. I played it for probably 10 hours straight once, through the night at a Christian Youth Group all-nighter thing. By the time my dad came to pick me up he next morning, I was literally thinking 'hold RB to board enemy vehicle' when I saw the car...